Measuring the magnetic flux surfaces of Wendelstein 7-X

The division E4 is operating a diagnostic which allows an accurate determination of the flux surfaces of Wendelstein 7-X. It works by emitting a thin filament of energetic electrons into the magnetic field. This current will trace the magnet field lines, while exciting visible fluorescence radiation in some molecules of the residual background gas. Thus, the magnetic field lines become 'visible'. First experiments yielded an excellent verification of the magnetic field calculations done beforehand.

For more information see the press release.

Flux surface diagnostics: the photograph combines the tracer of an electron beam on its multiple circulation along a field line through the plasma vessel with the image points left behind by a fluorescent rod which has been moved through the image plane. The rod is moved quickly and is not visible due to the lengthy exposure time.

Photo: IPP, Matthias Otte

Flux surface diagnostics: the photograph combines the tracer of an electron beam on its multiple circulation along a field line through the plasma vessel with the image points left behind by a fluorescent rod which has been moved through the image plane. The rod is moved quickly and is not visible due to the lengthy exposure time.

Photo: IPP, Matthias Otte

The evidence: the fluorescent rod makes closed, nested magnetic surfaces visible – the magnetic field cage for the plasma is exactly as it should be.

Photo: IPP, Matthias Otte

The evidence: the fluorescent rod makes closed, nested magnetic surfaces visible – the magnetic field cage for the plasma is exactly as it should be.

Photo: IPP, Matthias Otte

Injection of positrons into the field of a permanent magnet

In order to produce a magnetically confined pair plasma from electrons and positrons, positrons have to enter the magnetic confinement region from the outside. The PAX/APEX team reached this important milestone in a joint experiment with the positron group from TU Munich. Positrons from the NEPOMUC beamline at the FRM II research reactor in Garching where steered into the field of a permanent magnet. Up to almost 40 % of the initial particles were able to enter this field, and were stored there for on the order of 5 ms.

Results were published in 'New Journal of Physics', granting open access to all.

Simulated trajectories of positrons (blue) injected into the field of a permanent magnet. The positron beam approaches the magnet from the left.

IPP, H. Saitoh

Simulated trajectories of positrons (blue) injected into the field of a permanent magnet. The positron beam approaches the magnet from the left.

IPP, H. Saitoh

Stored positrons vs. confinement time, for positrons injected and stored in the field of a supported permanent magnet.

IPP, H. Saitoh

Stored positrons vs. confinement time, for positrons injected and stored in the field of a supported permanent magnet.